1. Field of the Invention
The present invention relates to photolithography. More particularly, the present invention relates to a projection lens of an exposure apparatus of a photolithography system, and to the associated phenomenon of flare that causes defects in a pattern formed on a wafer by light focused on the wafer by the projection lens.
2. Description of the Related Art
In general, flare is a phenomenon that produces a bad exposure in a photolithographic process due to defects of a projection lens of the exposure apparatus of the photolithography system. More specifically, when a portion of the surface of the lens is defective, the exposure light is dispersed at the defective portions, and photoresist patterns are formed incorrectly by the exposure light. Here, the defects at the surface of the lens which may produce flare include contaminants, scratches, or a difference in refractory indices between portions of the lens. Light passing through such defective portions of the lens during exposure scatters and thus, the light does not focus properly on the photoresist layer.
The flare phenomenon will be described more fully with reference to FIGS. 1 and 2. Referring to FIGS. 1A and 1B, an exposure apparatus for performing a photolithographic process includes a lens 14 for scaling down and projecting a light shielding pattern 11 of a mask 10 onto a predetermined portion of a wafer 12. The lens 14 is interposed between the mask 10 and the wafer 12. The top surface of the wafer 12 is coated with a photoresist layer (not shown).
As shown in FIG. 1A, if no defect occurs at the surface of the lens 14, the shielding pattern 11 on the mask 10 is projected on a reduced scale onto the photoresist layer. Accordingly, photoresist patterns 12a are formed on the wafer 12.
On the other hand, as shown in FIG. 1B, if defects occur at the surface of the lens 14, light disperses at the defective portions 15 of the lens 14. The dispersion of light results in an irregular distribution of light on the photoresist layer during the exposure, and decreases the contrast of the image. In addition, portions of the wafer 12 corresponding to and adjacent to the defective portions 15 may become excessively exposed. As a result, an on chip variation phenomenon occurs in which photoresist patterns 12b on the wafer 12 are deformed, or the widths of photoresist patterns 12b formed in one field vary. As the photolithographic process is repeated, the lens 14 of the exposure apparatus becomes more severely defective, and the amount of flare varies.
Accordingly, in photolithography, the amount of flare of a lens and the position on the wafer which is affected by flare must be measured and determined for every exposure process if the photoresist patterns are to be formed as desired.
However, conventional photolithography systems do not have tools for identifying whether flare is produced by a projection lens, for identifying whether a wafer is affected by flare, and/or for determining the extent of a flare-affected region on a wafer. Thus, it is difficult to correct for the flare, i.e., to avoid bad exposures.